The problem as I see it, is the suspension layout and motion ratios need to be decided at the initial chassis and suspension linkage design and planning stage. But most of us have no idea within a fairly wide margin, what the final all up sprung weight of the car is going to end up being. So how can you possibly do any spring design ?
You can do it, even if you have absolutely no idea what the final weight is going to be, but it requires a few logical steps and a few design decisions along the way. Here is how I am doing mine.....
First step is to decide the function of the vehicle, competitive off road track racer, to pure recreational street, covers a wide range. So that is the first thing you need to decide. How stiffly you really want it to ride on the springs, and what suspension travel you really need to have built into it.
Next step is to establish some initial suspension ride frequencies. These will usually end up somewhere between 1Hz (extreme comfort) to 2Hz (factory sports car) or even higher for serious racing on smooth tracks. But somewhere around 1.5 to 2.5 Hz would probably suit most of us here for a dual purpose car. But the whole thing is wide open to personal preference.
Once you have decided on the required suspension frequencies, here is where the magic comes in. There is a very simple formula to work out the static spring compression (at the wheel), for any required resonant suspension bounce frequency. And that is: frequency in bounces per minute = 188 divided by the square root of sprung deflection in inches at the wheel.
If you jack up your car at one corner so the spring just rattles free (theoretically), then lower it onto the ground and it sinks (say) five inches at the wheel, that is your static sprung deflection. Five inches of wheel travel. It does not matter if the weight on that particular wheel is 200 Lbs or 1,000 Lbs if it sinks five inches on it's spring at that corner, the suspension bounce frequency will always be the same.
Using the above formula, bounces/minute = 188 divided by square root of 5 inches 188 / 2.236 = 84 bounces per minute = 1.40 Hz which is fairly soft by some people's standards, but should give a good ride.
Maybe you prefer it a little firmer, but let us use five inches static sprung wheel compression as an example here.
Next you design your suspension links and decide on a suitable suspension travel, maybe three inches up and three inches down from ride height ? Or whatever you want it to be. And you design your coil shock mountings and linkages, and angles, to give you that six inches of total design travel at the wheel.
Once you know where and how the coil shocks mount, you can work out the motion ratio at ride height, and the total coil shock travel required to get the design plus and minus three inches of wheel travel.
Let's assume the motion ratio stays at exactly two, (to keep it simple). Total shock travel will be three inches for six inches of total wheel movement, and the shock will be exactly in the middle of it's range at ride height. Now you measure the required height of the spring on that coil shock at the mid point of it's travel, because that is where we want it to be at ride height.
Let's assume the spring measures exactly six inches long at ride height, with the final suspension design as it now stands. We know the wheel can only actually droop three inches, because that is how we designed it, but the design static sprung compression at the wheel needs to be the full five inches to get our required 84 bounces per minute (1.4Hz) ride frequency.
So what we need to figure out next, is the free length of the spring with the suspension theoretically at five inches of droop, even though the shock will not allow it to actually travel down that far. You can do it on a scale drawing, or measure it on the car with one end of the shock disconnected.
As we know our motion ratio is two, if the suspension linkage was completely linear, the spring would need to compress two and a half inches for the wheel to move up five inches. That is unlikely to be exactly correct if the linkage is non linear, but measure what the spring length should be at five inches of droop.
If we find the motion ratio remains at exactly two, then our spring needs to compress two and a half inches under load to get our five inches of static sprung wheel travel. As we know the spring at loaded ride height is going to be six inches, we order springs with a free length of 2.5 plus 6 inches.
Without knowing the final weight of the car, you can design for the required suspension softness or stiffness to get the exact ride you want.
When the car is finally built, some scales, and knowing the motion ratios will get you close with spring poundages. and you can try some different springs that all have a free length of 8.5 inches. If it sits too high fit softer springs. If too low, get stiffer springs. With the springs fitted that give the correct design ride height, you will also end up getting the exact suspension ride frequencies you originally planned for.
This way you can design absolutely everything, and know exactly how it is going to all end up, without having the faintest idea what the final vehicle weight is going to be. The springs themselves can be the very last thing you need to adjust to get things going as originally planned.
One final note. With rising rate suspensions, or any non linearity of spring/wheel movement, all you need to know is the exact motion ratio at final ride height, and the extended spring length at zero wheel load to get the correct static sprung wheel compression travel. What the motion ratio is between those two points does not matter. The spring will settle to it's design length at ride height, at the ride height motion ratio. How it got there from full droop zero wheel load, through a possibly very non linear motion ratio, makes no difference.
